Tire curing bladder lubricant

ABSTRACT

An aqueous lubricating composition for tire curing bladders containing polydimethylsiloxane, methyl hydrogen or methyl trimethoxy silane, a surfactant and optionally a metal salt of an organic acid.

FIELD OF THE INVENTION

This invention relates to tire curing bladder lubricant compositions,tire curing bladders having an outer coating of such lubricantcomposition, and a method of curing tires utilizing such a coatedbladder.

BACKGROUND OF THE INVENTION

Conventionally, pneumatic rubber vehicle tires are produced by moldingand curing a green, or uncured and unshaped, tire in a molding press inwhich the green tire is pressed outwardly against a mold surface bymeans of an inner fluid expandable bladder. By this method the greentire is shaped against the outer mold surface which defines the tire'stread pattern and configuration of sidewalls. By application of heat thetire is cured. Generally, the bladder is expanded by internal pressureprovided by a fluid such as hot gas, hot water and/or steam which alsoparticipates in the transfer of heat for curing or vulcanizationpurposes. The tire is then allowed to cool somewhat in the mold,sometimes aided by adding cold or cooler water to the bladder. Then themold is opened, the bladder collapsed by removal of its internal fluidpressure and the tire removed from the tire mold. Such use of the tirecuring bladders is well known to those having skill in such art.

It is recognized that there is a substantial relative movement betweenthe outer contacting surface of the bladder and the inner surface of thetire during the expansion phase of the bladder prior to fully curing thetire. Likewise there is also a considerable relative movement betweenthe outer contacting surface of the bladder and the cured inner surfaceof the tire after the tire has been molded and vulcanized during thecollapse and the stripping of the bladder from the tire.

Unless adequate lubrication is provided between the bladder and theinner surface of the tire there is typically a tendency for the bladderto buckle, resulting in a misshaping of the tire in the mold and alsoexcessive wear and roughening of the bladder surface itself. Also, thebladder surface can tend to stick to a tire's inner surface after thetire is cured and during the bladder collapsing part of the tire curecycle. Further, air bubbles can potentially become trapped between thebladder and tire surfaces and promote tire vulcanizing defects due tolack of adequate heat transfer.

For this reason, it is a conventional practice to precoat the innersurface of the green or uncured tire with a lubricant in order toprovide lubricity between the outer bladder surface and inner tiresurface during the tire shaping and molding operation. Sometimes thelubricant has been called a lining cement. Conventionally, the innersurface of the green tire, which is typically a rubber gum stock, issimply spray-coated in a confined, ventilated, spray booth, with alubricant which might, for example, be based upon a silicone polymer.Other additives may also conventionally be utilized in the lubricantcomposition such as mica, polymeric polyols, cellulose ethers, clay suchas bentonite clay and the like. Some lubricants are solvent based andsome are water based. Often aqueous soap solutions are utilized. Manylubricant compositions have been taught in the art for such purposes.

However, a conventional practice of spray-coating the inner surface ofthe green tire with a lubricant composition can result in a relativelyhigh labor intensive operation which can add appreciably to the cost ofproducing the tire. The tire must be transported to and from the spraybooth and time must be allowed for the spray-lubricant coat to dry.Therefore, it is desired to provide an enhanced lubrication system as acomposition and use therefore for molding or shaping and curing of greentires.

DISCLOSURE AND PRACTICE OF INVENTION

In accordance with this invention, a lubricant composition is providedas the product of a mixture which comprises:

(A) about 20 to about 40, preferably about 25 to about 35, parts byweight polydimethylsiloxane characterized by having a viscosity in therange of about 12 to about 28, preferably about 15 to about 25, millioncentistokes at 25° C.;

(B) about 35 to about 70, preferably about 45 to about 60 parts byweight of at least one silane selected from (i) a methyl hydrogen silanecharacterized by having a viscosity in the range of about 20 to about 40centistokes at 25° C., (ii) dimethyl hydrogen silane characterized byhaving a viscosity in the range of about 80 to about 120 centistokes at25° C., and (iii) a methyl trimethoxy silane;

(C) optionally about 3 to about 12, preferably about 5 to about 10 partsby weight of a metal salt of an organic acid selected from at least oneof zinc, magnesium, manganese and cobalt acetates, stearates,propionates, glutionates and octoates; and

(D) about 10 to about 25, preferably about 15 to about 20 parts byweight surfactant(s).

The composition for application to the bladder surface is an aqueousemulsion, or dispersion of the composition. For example, the compositionfor such application also contains (E) about 500 to about 1500,preferably about 600 to about 800 parts by weight water which, afterapplication to the bladder, is dried by evaporation. Certainly morewater could be used, although additional dilution of the compositionshould reduce the efficiency of its application.

It is generally preferred that the polydimethylsiloxane of (A) ishydroxyl capped as an ingredient (precursor) in preparing thecomposition.

Generally, the methyl hydrogen silane or mixture of methyl hydrogensilane and dimethyl hydrogen silane, (e.g. in weight ratios of 20/80 to80/20), is preferred for (B).

Although the action of the metal salt of the organic acid is notcompletely understood, apparently it acts somewhat as a catalyst. Itsuse is optional since it has been observed that the lubricantcomposition could set up rather easily and adequately without thiscompound. Zinc acetate and/or zinc stearate is generally preferred forthis purpose, although magnesium, manganese and cobalt acetates shouldalso be suitable to a more or less degree as well as zinc propionate andzinc glutinate.

It is to be appreciated that various relatively well known defoamingagents and various stabilizers can also be used in the composition ofthis invention which are generally well known to those having skill inthe pertaining art.

In further accordance with this invention, an expandable rubbertire-curing bladder having such a coating composition thereon(particularly after water removal) is provided. In practice, the rubberfor the bladder is typically a butyl or butyl-type rubber (copolymer ofisoprene and isobutylene). By the term butyl-type, it is intended tomean various modified basic butyl rubbers such as halogen-substitutedbutyl rubbers which may be, for example, chlorobutyl or bromobutylrubber.

In still further practice of this invention, a method of preparing apneumatic or semi-pneumatic rubber tire is provided in which a greentire is placed in a tire mold, the coated expandable bladder of thisinvention positioned therein, the mold closed and bladder expanded byapplication of internal hot fluid pressure to force the tire outwardagainst the mold surface to shape and cure the tire followed by openingthe mold, collapsing the bladder and removing the shaped and cured tire.

In more detail, for example, such a method of molding a pneumatic orsemi-pneumatic tire which comprises the steps of:

(A) providing or building a green tire with elements which are to be itsouter tread for ground-contacting purposes, two spaced intensible beads,sidewalls extending radially outwardly from said beads to join saidtread, supporting carcass with reinforcing elements, and an innersurface of rubber gum stock;

(B) inserting said green tire into a tire mold press and positioning acoated tire cure bladder of this invention inside of said green tire,said bladder being attached to an internal portion of said tire press;

(C) closing the tire mold and expanding said coated tire cure bladder byinternal, heated fluid outwardly against the inner gum stock surface ofsaid tire to press the tire outward under conditions of heat andpressure to shape and cure said tire;

(D) opening the tire mold, collapsing said bladder and removing thecured tire having a generally toroidal shape therefrom.

The term "pneumatic tire" relates to tires which rely on an internalfluid, such as air under pressure in their tire cavity for their properoperation when mounted on a rim and the term "semi-pneumatic" tirerelates to tires which contain an internal fluid, such as air, in theircavity but do not totally rely on its pressure for its proper operationwhen mounted on a rim.

In the practice of this invention, the aqueous emulsion or dispersion ofthe lubricant composition can conveniently be provided by the methodwhich comprises:

(A) mixing 20 to about 40, preferably about 25 to about 35 parts byweight of the polydimethylsiloxane with about 35 to about 70, preferablyabout 45 to about 60 parts by weight of at least one of the methylhydrogen silane, dimethyl hydrogen silane or methyl trimethoxy silane,about 500 to about1500, preferably about 600 to about 800 parts byweight water (a portion of the water actually being added as a mixturewith the silane(s)) and optionally, about 3 to about 12, preferablyabout 5 to about 10 parts by weight of said metal salt of an organicacid (generally a portion of the water is pre-mixed with the salt);

(B) optionally mixing therewith about 0.2 to about 1.0 parts by weightdefoaming agent (which might be, for example, a defoamer silicone of thedimethylpolysiloxane emulsion in water); and

(C) optionally, mixing therewith about 2 to about 10 parts by weightstabilizer (for increased stability of the emulsion or dispersion).

The aqueous emulsion or dispersion is simply coated, such as byspray-coating, onto the bladder and dried by evaporation at atemperature, for example, in the range of about 20° C. to about 110° C.It is preferred that the bladder is from about 80 to about 150 percentof its tire curing expanded position or condition for this coatingpurpose (as opposed to being deflated or collapsed), although it is notconsidered necessary and bladders have been successfully coated in asomewhat deflated condition.

It should be pointed out that various silicone or siloxane-based aqueousemulsion or dispersion lubricant compositions were previously formulatedand evaluated as cure bladder coatings. Indeed, such a composition wasearlier tried containing a polydimethylsiloxane polymer or mixture ofsuch polymers of different molecular weights and viscosities. In suchcompositions it was found by experience that when utilizing a coatingthereof on a rubber bladder, only about four to six tires could bemolded from such bladder (4 to 6 tire cure cycles) for the singlesiloxane fluid composition or maybe 10 to 15 cycles from the dualsiloxane polymer composition until excessive adhesion between thecontacting outer surface of the bladder and the inner surface of thetire was experienced, as evidenced by their tendency to excessivelystick together upon collapse of the bladder after curing the tire.

Organic solvent based silicone compound-contacting lubricantcompositions were also evaluated. However, difficulties were experiencedbecause apparently the organic solvent in the composition eventuallydamaged or degraded the rubber surface of the bladder itself. Theorganic solvent itself presented potential flammability and toxicityproblems. Moreover, the evaulated organic solvent-based coatingcompositions on the cure bladder were observed to effectively last onlyabout 4 to 6 tire cure cycles until it was considered necessary oradvisable to recoat the bladder surface. Such frequency of recoats isconsidered of marginal economical benefit when taken or coupled with theattendant potential hazards.

Upon utilizing the coating composition of this invention on a tire curebladder, it was observed that about 14 to about 30 truck tires, andsometimes even more, could be shaped and molded with the bladder (or 14to 30 cure cycles) before excessive adhesion between the bladder andcured tire surfaces was experienced.

The practice of this invention is further illustrated by reference tothe following example which is intended to be representative rather thanrestrictive of the scope of the invention. Unless otherwise indicated,all parts and percentages are by weight.

EXAMPLE I

A lubricant composition was prepared according to the following recipeshown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Material                Parts                                                 ______________________________________                                        Polydimethylsiloxane, hydroxyl capped.sup.1                                                           40.4                                                  Methyl hydrogen silane (30%).sup.2                                                                    175.0                                                 Zinc acetate/stearate (20%).sup.3                                                                     43.7                                                  Defoamer.sup.4          1.3                                                   Water                   610.8                                                 ______________________________________                                         .sup.1 Reported to be a polydimethylsiloxane having a viscosity in the        range of about 20 million centistokes as a mixture comprised of 30.7 part     of the siloxane and 10.9 parts surfactant therefor.                           .sup.2 Reported to be a mixture consisting of (a) 30 weight percent of a      mixture of methyl hydrogen silane having a viscosity of about 30              centistokes at 25° C. and dimethyl hydrogen silane having a            viscosity of about 100 centistokes at 25° C. and (b) 70 weight         percent water.                                                                .sup.3 Reported to be a mixture comprised of (a) 20 weight percent of a       mixture of zinc acetate and zinc stearate and (b) 80 weight percent water     .sup.4 The optional defoamer can be of a dimethylpolysiloxane emulsion in     water composition which is beneficial because it prevents or inhibits foa     formation during mixing.                                                 

The aqueous emulsion or dispersion was prepared according to thefollowing method:

(A) Add the polydimethylsiloxane to the water with relatively low shearmixing at 1000 rpm in a Cowles mixer to form a water emulsion.

(B) Add the defoamer.

(C) Slow mix at 500 rpm and add the methyl hydrogen silane and dimethylhydrogen silane aqueous mixture.

(D) Add zinc acetate/zinc stearate aqueous mixture.

The mixture was sprayed on the outer surface of the rubber tire curingbladder in its somewhat collapsed condition. The coating was allowed todry at about 80° C. The coating was re-applied after about 10 to 15 tirecure cycles with maximum of about 24 hours between coating applications.

Upon spraying the mixture on a hot bladder surface, the methyl hydrogensilane mixture apparently reacts with the polydimethylsiloxane to form asomewhat cured silicone polymer on the surface of the bladder.

The coating was dried for about a minute on the hot (80° C.) bladdersurface to form the lubricant composition coating thereon.

The bladder itself was of the butyl rubber type of a generally toroidalshape with an overall, expanded condition diameter of about 39 inchesand tubular diameter of about 10 inches. Its surface had beenpre-treated by washing with a hydrocarbon solvent, followed by drying,to remove surface oils and the like.

A radial ply green tire was fabricated of the 11 R 22.5 size.

The tire was placed in a tire mold press and the coated bladder,attached to the mold, inserted inside the tire. The mold was closed andthe bladder was expanded by steam at a temperature of about 190° C. toforce it against the inside surface of the tire and press the tireoutwardly against the outer mold surface so that the tire was shaped asdesired and cured.

The mold was then opened, the bladder collapsed and the tire removedtherefrom. By this procedure it was observed that about 20 to about 30tires could be molded from the bladder within about a 24 hour period(cure cycles) before it was necessary to recoat the bladder with thelubricant composition.

Generally, recoating is considered necessary when the bladderexcessively sticks to the inner surface of precured tire when theexpanded bladder is collapsed or collapsing after the curing operation.

It is recognized that the inner surface of the tire is typically acompounded rubber gum stock which can be of various rubber or theirmixtures such as natural rubber, cis 1,4-polyisoprene, cis1,4-polybutadiene, butadiene-styrene copolymer, butyl rubber, holobutylrubber such as chlorobutyl and bromobutyl and EPDM(ethylene-propylene-minor amount of diene terpolymer).

In this example, preparation of a radial ply tire is exemplified.Although the invention can sometimes be considered to be more adaptableto radial ply rather than to bias ply tire production because thebladders are generally required to expand more during a bias ply tirecure cycle, thereby stressing the bladder's surface coat, it isconsidered that the invention is generally readily adaptable to bias plytire production.

It is important to appreciate that the tire in this Example was preparedby expanding the coated bladder directly against the inner gum stocksurface of the tire to press the tire outward under conditions of heatand pressure to shape and cure the tire. Thus, the coated bladdereffectively enabled the preparation of the tire without application of alubricant coating or liner cement, on the inner surface of the greentire. This is considered significant since it adequately demonstratedthat the coated bladder of this invention provided adequate lubricationfor a series of sequential tire cure cycles under conditions of heat andpressure, expansion and contraction without the conventional, attendanttire liner cement, or lubricant, pre-coated on the inside surface of thetire. It is reasonably considered that this will result in aconsiderable savings of labor and material in the preparation of apneumatic tire. Although it is appreciated that a lubricant pre-coatcould be used on the inner surface of the green tire, if desired, inconjunction with the coated bladder, it is considered important thatthis Example demonstrated that it was not required.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

What is claimed is:
 1. A lubricant composition comprising an aqueousemulsion consisting essentially of:(i) about 20 to about 40 parts byweight polydimethylsiloxane characterized by having a viscosity in therange of about 12 million to about 28 million centistokes at 25° C.;(ii) about 12 to about 20 parts by weight of at least one silaneselected from (a) a methyl hyrogen silane, characterized by having aviscosity in the range of about 20 to about 40 centistokes at 25° C.,(b) dimethyl hydrogen silane characterized by having a viscosity in therange of about 80 to about 120 centistokes at 25° C. and (c) methyltrimethoxy silane; (iii) optionally, about 3 to about 12 parts by weightof a metal salt of an organic acid selected from at least one of zinc,magnesium, manganese and cobalt acetate, stearate, propionate andoctoate; (iv) about 10 to about 25 parts by weight surfactant for saidpolydimethylsiloxane and (v) about 500 to about 1500 parts by weightwater.
 2. The lubricant composition of claim 1 where saidpolydimethylsiloxane is hydroxyl capped.
 3. The compositionn of claim 2where the silane is methyl hydrogen silane and/or dimethyl hydrogensilane.
 4. The composition of claim 1 or 2 where said metal salt is zincacetate and/or zinc stearate.
 5. The lubricant composition of claim 2where the silane is methyl hydrogen silane and/or dimethyl hydrogensilane and the metal salt is zinc acetate and/or zinc stearate.